In the sector of pre-press activity known as graphic and reprographic arts an intensive use is made of contact copying materials to reproduce screen dot images, line work and typesetting work. Both negative working photographic materials which produce negative-positive or positive-negative copies are used as well as so-called direct positive working materials giving rise to negative-negative or positive-positive reproductions.
In order to obtain exact copies with sharp dot and line edges, it is necessary to use fine-grained relatively insensitive photographic emulsions. The materials containing this type of emulsions are image-wise exposed in contact with the original in a graphic arts copying apparatus by means of high intensity radiation, preferably by light sources emitting a high content of near-ultraviolet light. Common light sources for this purpose are mercury vapour lamps, metal-halogen lamps, xenon tubes, pulsed xenon tubes and quartz-halogen sources.
The handling of ever increasing amounts of photographic materials of different kinds, the decentralisation of the distinct steps in the reproduction cycle etc., have created a demand for silver halide materials which can be handled under clear ambient light illumination. This demand has given rise to the development of so called "roomlight materials" which can be image-wise exposed, handled and processed without the occurence of fogging in a reasonable time while being illuminated by common office fluorescent tubes and daylight penetrating through office windows. Prior art material which can be handled under roomlight conditions has been described in e.g. U.S. Pat. No. 2,219,667 and GB 1,330,044.
A much appreciated photographic effect occuring with some commercially available roomlight materials is the so-called "print-out" effect. This phenomenon can be generally described as the darkening of photographic emulsions under the influence of extended exposure to radiation or of image-wise exposure to a high intensity source followed by a low intensity exposure if required. Depending on the application the print-out image can be further developed to the full metal silver density. Several commercial roomlight materials show a print-out effect after image-wise exposure by a common UV emitting light source. Since roomlight emulsion are handled in bright light, this print-out effect allows visual differentiation between exposed and non-exposed parts. This is much appreciated by the customer handling roomlight contact copying materials because it allows him to check before processing if an exposure has been made of the correct original and if it has been made at the correct dimensional position on the copying material sheet. The latter is most important in the case when successive exposures are to be made of several originals resulting in an assembly of copies on one roomlight material sheet. So it is clear that the presence of the print-out effect is an important aspect of convenient handling. However it is a disadvantage that with most present roomlight materials the print-out effect becomes clearly visible only after considerable overexposure.
Scientific reports on the print-out effect have been published in e.g. J. Photogr. Sci., Vol. 6, p. 141 (1958), J. Opt. Soc. Am., Vol. 28, p. 431 (1938), J. Chem. Phys., Vol. 18, p. 499 (1950), J. Opt. Soc. Am., Vol. 23, p. 157 (1933) and ibid., Vol. 26, p. 347 (1936). X-ray or electron diffraction demonstrate that silver metal is formed; so the print-out effect can be conceived as a latent image photolytically grown to visible dimensions. Analytical techniques show the release of halogen; so it is no wonder that so-called halogen acceptors can enhance the print-out effect. Suitable halogen acceptors which promote the print-out effect are disclosed e.g. in U.S. Pat. Nos. 3,287,137 and 3,364,032. The print-out effect can serve merely as a control method as it is the case with roomlight copying materials where a conventional development follows, or it can constitute the final image as it is the case e.g. in high speed oscillographic recording materials. Suitable halogen acceptors for this kind of application are e.g. perhydro-1,2 4-triazine-5-one-3-thione derivatives disclosed in U.S. Pat. No. 3,502,471.
Another aspect of convenient handling for the consumer of roomlight materials, apart from the print-out effect, is the presence of sufficient development latitude. One way of defining the development latitude is the development time interval, expressed as maximal procentual deviation from the ideal development time, in which the sensitometric properties and the screen dot quality remain at an acceptable level.
In the pre-press graphic arts field one can distinguish three major groups of processing systems. Widely used are the so-called Rapid Access systems (type 1) which employ a superadditive mixture of two developing agents e.g. hydrochinon and 1-phenyl-3-pyrazolidinone or hydrochinon and N-methyl-p-aminophenol; they contain sufficient sulphite ions to provide for stable developer solutions, resistant to exhaustion and aerial oxidation. An additional advantage is their broad development latitude. However these systems do not provide for superior dot quality and are restricted to those applications where superior dot quality is not utmost important. Another classical processing systems group comprises the so-called "lith developers" containing hydrochinon as sole developing agent and a low sulphite concentration (type 2), which produce excellent dot quality but are more expensive and less stable and therefor require sophisticated regeneration methods in order to compensate for exhaustion and aerial oxidation. Of recent years a third group of graphic arts processing systems came into existence combining the advantages of type 1 and type 2 (stability and dot quality). This new, so-called "hard-dot Rapid Access" systems combine a high sulphite content with a mechanism in which a chemical speices is sufficiently active to initiate an infectious development or an infectious development-like high-contrast development. Possible mechanisms can be based on, but are not restricted to hydrazine, hydrochinon or tetrazolium salt chemistries. These systems have the superior quality of classical "lith" systems in addition to the good chemical stability of the conventional Rapid Access systems. Examples of these type 3 developing systems and compatible materials are marketed by EASTMAN KODAK Co under the trade name ULTRATEC, by FUJI PHOTO Ltd under the trade name GRANDEX and by AGFA under the trade name AGFASTAR. An example of a developer as used in the latter system contains essentially hydrochinon as sole developing agent, sufficient sulphite ions comparable to type 1 developers, a nitroindazole derivative, a polyalkyleneoxide polymer and an inorganic alkali, as disclosed e.g. in U.S. Pat. No. 4,710,451.
When using graphic arts emulsions containing bromide as major halide the development latitude is very large (from about 10 to 60 seconds) in type 1 developers. In lith type developers (type 2) this latitude is reduced to about 10% which is no easy-to-handle situation. On the contrary, in type 3 developers a satisfying situation of about 50% development latitude has been restored. However for roomlight applications the emulsion designer tends to prefer silver chloride emulsions which become less easy fogged under ambient light conditions than bromide emulsions thanks to their spectral sensitivity which has its maximal value in the near ultraviolet and extends less in the visible spectral region than it is the case with bromide emulsions. However silver chloride emulsions tend to develop faster than silver bromide emulsions particularly in "hard-dot Rapid Access" developers. So for the convenience of compatibility of development time of both chloride and bromide emulsions in the same processing sequence, development restrainers have to be added to silver chloride emulsions. However these development restrainers show the disadvantage of strongly reducing the development latitude especially in developers of type 3.
It is the object of the present invention to overcome this problem of reduced development latitude when roomlight materials are processed in "hard-dot Rapid Access" developers.
It is a further object of the present invention to realise graphic arts photographic roomlight materials which exhibit a clear print-out effect, not only at overexposure but at the correct exposure to reproduce exactly a 50% screen dot.